Given a class symbol cls, construct a name representing this constructor.

Given a class symbol cls, construct a name representing this constructor.
For a class:

a.b.Foo

this is:

a;b;Foo;init;

The prefix is important to avoid name hashing all constructors together
(see #97), the weird format is necessary to avoid scalac or zinc trying to
interpret this name (in particular we should not use '.' and we should not
use '<init>'), we use ';' because it is one of the few characters that
cannot appear in a valid JVM name.